[InstCombine] When checking to see if we can turn subtracts of 2^n - 1 into xor, we only need to call computeKnownBits on the RHS not the whole subtract. While there use isMask instead of isPowerOf2(C+1)

Calling computeKnownBits on the RHS should allows us to recurse one step further. isMask is equivalent to the isPowerOf2(C+1) except in the case where C is all ones. But that was already handled earlier by creating a not which is an Xor with all ones. So this should be fine.

llvm-svn: 299710

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